This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the
pathway.
If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

Polyketide (exclusive of fatty acids) represent a large group of natural products, i.e. secondary metabolites that are widespread in fungi, bacteria and plants [Birch68, Schroder99, Wink03]. Polyketides are often involved as intermediates in the formation of a vast variety of secondary metabolites with mixed biosynthetic origin [Schroder99a] [Schroder97b].

The entry enzymes of the biosynthesis of polyketides in plants are the polyketide synthases producing the general backbone of compounds who give rise to a large array of natural products representing an amazing degree of structural diversity. The chalcone synthase (CHS) and stilbene synthase (STS) are the most well-known plant polyketide synthases, however there exists a growing superfamily of related proteins referred to as CHS/STS-type proteins [Schroder99]. Depending on the specific polyketide synthase involved very diverse compound classes evolve such as chalcones (flavonoid biosynthesis), stilbenes [Schroder99a], stilbenecarboxylates [Eckermann03], benzalacetone [BorejszaWysocki96], pyrone [Eckermann98], C-methylated chalcone [Shen00] and acridone alkaloids [Baumert94] [Junghanns95].

The formation of polyketides resembles the biosynthesis of fatty acids but is different in various aspects. In contrast to fatty acid synthases (FAS) plant polyketide synthases (PKS) are relatively small homodimeric enzymes, which do not use the acyl carrier protein (ACP) as activating agent but utilize acyl-CoA esters directly as substrates [Shen00] [Birch68]. Although the synthesis of chalcone usually does not involve modifications by other proteins the chalcone derivatives reduced at a specific position in the synthesized aromatic ring are frequently found in plants increasing the structural variability of derivatives [Schroder97b]. A number of diverse type III polyketide synthases present in various plants have been cloned and found to produce diketides and heptaketides [Abe04] as well as pentaketide chromones [Abe05].

About This Pathway

The pentaketide chromone synthase (PCS) involved in the biosynthesis of 5,7-dihydroxy-2-methylchromone represents a novel type of PKSs catalyzing five successive condensations steps with malonyl-CoA as the starter unit. The product is an aromatic pentaketide chromone. The pentaketide chromone synthase (EC 2.3.1.-) isolated from aloe (Aloe arborescens) belongs to the CHS superfamily (PKS type III). The pentaketide chromone synthase is a plant-specific, unique enzyme taking up a central function in the biosynthesis of chromones [Abe05].